In the field of electrostatic actuators, a device can be formed of repeating layers of structural, sacrificial, and dielectric materials which are patterned and stacked to form complex three dimensional structures. Electrostatic actuators can typically include a lower electrode opposed by a deformable upper electrode. In order to arrive at such a structure, the lower electrode can be patterned to include isolation gaps between adjacent electrode structures. A sacrificial material can be layered on the lower patterned electrode prior to depositing the upper deformable electrode.
It is a problem in the art, however, that the sacrificial material flows into and conforms to a surface variation of the isolation gaps or “cuts” in the electrode. When the upper electrode is deposited, the surface variation mimics that of the sacrificial material and in some instances can even become exaggerated. The flowing of the sacrificial material into the isolation gaps therefore causes a gap between the spaced electrodes to have a smaller distance therebetween at the location of the isolation gap. This coupled with a known field concentration at the corner of the cut in the electrode, combine to make the location a very likely target for air breakdown, killing the device, or at least changing its behavior over time. In addition, the corners can cause problems in subsequent depositions. For example, a lip can form in a subsequent layer on a high end of the cut, the lip increasing in size over multiple depositions. When the top electrode is deposited, it fills these cracks and results in very sharp protrusions, which resemble stalactites. It is these “stalactites” which can short the device, causing premature breakdown or at least changing device behavior over time.
Current solutions to the problem include chemically mechanically polishing (CMP) any excess of the deposited sacrificial material, thereby filling holes in the bottom film and eliminating the topography of the sacrificial material. However, CMP is an expensive and potentially dirty process. Accordingly, alternatives to CMP are sought.
Thus, there is a need to overcome these and other problems of the prior art and to provide a method and apparatus for minimizing electric field concentration in MEMS devices, particularly at an edge of an isolation gap of a patterned electrode.